The present disclosure relates to a multilevel inverter capable of replacing a chemical capacitor with a film capacitor.
In general, a large high voltage induction motor is variably designed with voltages ranging from 2,400v to 7,200v, while a high voltage inverter which is a variable speed motor device suffers from drawbacks such as increased cost, need of wide area for installation and decreased system efficiency due to application to various motors using separate step-up and step-down transformers and also due to lack of variable voltages. The high voltage inverter also suffers from drawbacks such as harmonic influence on bus, motor burnt-out and vibration caused by pulse width modulation voltage.
Various kinds of multilevel inverters have been developed to overcome the drawbacks, and one of the power topologies showing the most excellent characteristics in terms of input/output quality is an H-bridge multilevel inverter. The H-bridge multilevel inverter generally a multilevel inverter using a cascade configuration which is a multilevel topology of a high voltage and large capacity inverter, in which several single phase inverters (hereinafter, referred to as power cells or cells) are connected in series for each phase of a three-phase current and accordingly a high voltage can be obtained by using low voltage power semiconductor switches within the power cells. Thus, the H-bridge multilevel inverter is called a cascade inverter.
The H-bridge multilevel inverter using a cascade configuration has a feature of a pulse width modulation/phase shift wherein a phase difference is sequentially generated between power cells which are serially connected to one another. Accordingly, the H-bridge multilevel inverter can have a low rate of output voltage change (dv/dt). In addition, the multilevel inverter using the cascade configuration can obtain reduced total harmonic distortion (THD) due to an output voltage with multi levels, namely, many steps.
Further, the H-bridge multilevel inverter using the cascade configuration rarely incurs a voltage reflection. Accordingly, in spite of a long distance between the multilevel inverter using the cascade configuration and a motor, there is no need for a separate device to prevent the voltage reflection phenomenon.
Unlike other multilevel inverters, the H-bridge multilevel inverter has advantages such as no issues of voltage imbalance between DC-link capacitors and easy extension to a desired output voltage by modularization.
FIG. 1 is a circuit diagram illustrating a configuration of a conventional H-bridge multilevel inverter system.
Referring to FIG. 1, the H-bridge multilevel inverter system consists of a plurality of power cells 2 each connected in series, where the plurality of single phase power cells 2 are connected in series for each phase of a three-phase current and each power cell 2 has an independent single phase inverter structure. An input unit connected to a power system is a transformer 6 having several tabs of extended delta connection method at a secondary wiring side.
FIG. 2 is a circuit diagram illustrating configuration of power cell of the conventional H-bridge inverter.
Referring to FIG. 2, the H-bridge inverter comprises: an input alternating current (AC) power source 10; a converter unit 11 converting the inputted AC power source to a direct current (DC) power source; an initial charging resistor 12 preventing an inflow of a rush current during input of the inputted AC power source; an electronic contactor 13 separating the initial charging resistor 12 from the circuit following the prevention of the rush current; a chemical capacitor 14 rectifying a DC voltage; an inverter unit 15 converting the inputted DC power source in response to a pulse width modulation (PWM) control signal; a current detector 16 detecting a current outputted from the inverter unit 15; a power cell main controller 17 collecting various information including a three-phase current and the DC voltage of the inverter unit 15 and exchanging various instructions and information with a master controller (17. not shown); and a PWM controller 18 receiving voltage instruction and frequency instruction from the power cell main controller 17 to generate a pulse width modulation (PWM) control signal.
Each power cell of the convention H-bridge inverter is equipped with the chemical capacitor 14. The chemical capacitor 14 is an essential constituent element having a direct influence on the life of the inverter, such that the chemical capacitor 14 should be carefully chosen as it affects greatly on a current ripple rate and ambient temperature.
In general, the chemical capacitor 14 functions in the following manner. That is, the chemical capacitor 14 serves to compensate an instantaneous difference between an input power and an output power for each power cell unit, to compensate an output using an energy of the chemical capacitor 14 for a predetermined time period during instantaneous black-out, and functions to store a regenerative energy when the regenerative energy is generated.
However, the high-voltage inverter system which is a single phase output inverter system suffers from a drawback of being installed with more numbers of capacitors than those of the three phase output inverter, thereby increasing the overall size of the entire system.
The chemical capacitor 14 has a high volume capacitance, where a current inputted into a DC terminal generally shows a rabbit ear-shaped discontinuous current, which is a cause of generating harmonics.
Still another drawback of the chemical capacitor 14 is that circuits associated with initial charging, for example, circuits such as initial charging resistor 12 and electronic contactor 13, are additionally required.